Gap control in a conventional electric discharge machining apparatus is performed so that a predetermined reference average voltage is provided and a gap average voltage of a supply voltage is approximated to this reference average voltage. FIG. 6 is an explanatory diagram showing a relation among a gap voltage, the gap average voltage and a gap dimension in such conventional gap control, and FIG. 6(a) shows the gap voltage, and FIG. 6(b) shows the gap average voltage, and FIG. 6(c) shows the gap dimension. In the case (for example, corresponding to an interval T2 in the drawing) that a work product such as work scraps accumulates in the gap and an electric discharge tends to occur, the gap average voltage becomes small as shown in FIG. 6(b) even when the gap dimension does not become small, so that control is performed in the direction that increases the gap dimension as shown in FIG. 6(c). Thus, the accumulation of the work product such as work scraps in the gap becomes a factor in reducing machining accuracy of a machining depth and so on.
Particularly, in powder mixture electric discharge machining, there was a problem that when machining proceeds with an increase in a work product such as work scraps in the gap left standing since the machining is stable, a gap dimension becomes too large and even in the case of machining on the same machining condition, a machining depth or a clearance of the side varies largely. In the powder mixture electric discharge machining, there is a report that the gap dimension becomes, for example, the order of 15 μm to 50 μm with respect to variations in the gap dimension due to accumulation of the work product such as work scraps in the gap.
One example of a work scrap concentration of a gap is calculated with respect to the case of finishing machining with an extremely low machining speed (the case that an electrode area is 100 cm2 and a gap dimension is 10 μm and a peak value Ip of an electric discharge current is 1 A and an average current value I is 0.2 A and a workpiece (negative polarity) is steel and an electrode (positive polarity) is copper). Work scraps W are calculated using W≅0.0097.Ip.I(g/min) which is an experimental formula and when it is assumed that volume of machining liquid of the gap is V (1), a work scrap concentration CON of the gap after 1 minute of machining becomes CON=W/V=(0.0097×1×0.2)/(100×10×10−7)=19.4(g/l). This calculation example indicates that the work scrap concentration of the gap increases in a short time regardless of the case of the finishing machining with an extremely low machining speed. Also, since a carbon product by electrode consumption scraps and decomposition of the machining liquid is added to this work scrap concentration CON, an actual concentration becomes several times the calculation value CON. Due to such an increase in the concentration of work scraps etc. in the gap, the gap dimension becomes large and machining accuracy reduces.
Also, the so-called electrode jump operation in which ejection of the work scraps etc. and replacement of the machining liquid of the gap are performed by rapidly jumping the electrode with respect to the workpiece at a rate of one time per several seconds to several tens of seconds is performed, but since a jump interval is determined by experience of individual operators, there was a problem that a reduction in machining productivity is caused since machining pauses during the jump operation when the jump operation is performed with frequency higher than required and a reduction in machining accuracy is caused when the jump operation is performed with low frequency.
An electric discharge machining method by powder mixture machining liquid for actually measuring a gap dimension is disclosed in JP-A-3-251322, and an electric discharge machining apparatus for measuring rise time of a pulse-shaped voltage and detecting a gap dimension by calculation from an electrode area and a dielectric constant of a gap, etc. is disclosed in JP-A-4-35811. When control is performed so that the gap dimension detected thus is approximated to a predetermined value, for example, even in case that an electric discharge has become unstable due to some factor, information that the electric discharge is unstable cannot be obtained in control by only the gap dimension and as a result, there was a problem that machining accuracy, particularly a surface property reduces.